Parasitic copepod (Ergasillus sp.). Coloured scanning electron micrograph (SEM) of a parasitic copepod (centre) in the gill of a fish. Copepods are microscopic aquatic crustaceans found in waters around the world. They may either be free living or parasitic on other organisms. They are the main constituent of zooplankton and are therefore a very important food source for other marine animals. Magnification: x67, when printed 10 centimetres wide.